What is a Pockels Cell?

A Pockels cell is an optical device that changes the polarization of light passing through it using an electric field. The Pockels cell consists of a crystal, often potassium di-deuterium phosphate (KD*P) or β-barium borate (BBO), known for their electro-optic properties. When a voltage is applied between the electrodes of Pockels cell, the electric field induced within the crystal causes a change in its refractive index. This change is proportional to the applied electric field strength. By adjusting the strength and orientation of the electric field applied to the crystal, the Pockels cell effectively controls the polarization state of the light passing through it. 

The crystal is isotropic along the optical axis, so that, when linearly polarized light passes in the direction of the optical axis, the polarization remains unchanged. However, the application of an external electrical field induces birefringence in the crystal, leading to a phase shift in the propagating light wave. Consequently, a phase of the light beam can be accurately and rapidly modulated by an electric signal.

In 1893, German physicist Friedrich Carl Alwin Pockels, through his studies on the Pockels effect, invented the cell. This cell has become vital in modern photonic applications, including ultra-fast lasers and precise light modulators. When combined with polarizers, these cells have the ability to operate as optical switches or laser Q-switches.

Pockels cells can be categorized into two main groups based on the orientation of the electric field. They are longitudinal devices and transverse devices.

Longitudinal Pockels cells are devices that have the electric field aligned in the same direction as the light beam, allowing for high apertures, and are commonly utilized in Q-switches and light shutters. 

Longitudinal Pockels cell

Transverse Pockels cells are devices that have the electric field passing perpendicular to the light beam, making them suitable for smaller apertures while requiring lower switching voltages. 

Transverse Pockels cell

In transverse devices, the half-wave voltage is determined by the distance between electrodes, with larger apertures necessitating higher voltages. The key parameter to consider is the half-wave voltage (Uπ), which signifies the voltage needed to induce a phase shift of π, or half the optical length. When used as an amplitude modulator, the half-wave voltage determines the transition from minimum to maximum transmission in the system. Both longitudinal and transverse Pockels cells typically operate with half-wave voltages ranging from a few hundred to several thousand volts.

However, certain highly nonlinear crystal materials like LiNbO₃ and integrated optical modulators with closely spaced electrodes allow for relatively smaller half-wave voltages. Nonetheless, these devices come with limitations in their power handling capacity.

Pockels cells commonly employ nonlinear crystal materials such as potassium di-deuterium phosphate (KD*P), potassium titanyl phosphate (KTP), β-barium borate (BBO) for higher average powers and/or higher switching frequencies, lithium niobate (LiNbO₃), lithium tantalate (LiTaO₃), and ammonium dihydrogen phosphate (NH₄H₂PO₄, ADP). Special materials like cadmium telluride (CdTe) are necessary for mid-infrared applications.

 Factors that influence the selection of materials include:

•  A high electro-optic coefficient to minimize the required drive voltage.
• Transparency within the necessary spectral range.
• Minimal residual absorption, high damage threshold, and low optical nonlinearities to support operation at high intensity levels.
• Accessibility to sufficiently large, high-quality crystals at reasonable costs.
• Low dielectric susceptibility to minimize electrical capacitance.
• Limited susceptibility to ringing effects, unwanted oscillations or fluctuations in the electrical response of the cell after the application of a voltage signal.

Applications of Pockels cell

Pockels cells find applications in various fields, including:

• Q-Switching: Pockels cells are crucial components in Q-switched lasers. By rapidly modulating the polarization of light within the laser cavity, these cells enable the generation of high-energy, short-duration pulses. This technique is widely used in applications such as laser engraving, range finding, and material processing.
• Regenerative Amplification: Pockels cells are employed in regenerative amplifiers to control the amplification of ultrashort pulses. They help in the precise timing of pulse amplification, ensuring the pulses are amplified only when needed. This is essential in high-power laser systems used for scientific research and industrial applications. 
• Pulse Picking and Amplifier Isolation: Pockels cells are used for selectively picking individual pulses from a continuous laser beam. Additionally, they provide isolation between different stages of amplifiers in high-energy laser systems. This ensures that the pulses remain well-defined and separated, enabling applications in fields like laser spectroscopy and particle acceleration. 
• Pulse Slicing and Removal of Pre-Pulse/ASE (Amplified Spontaneous Emission): Pockels cells are employed to slice pulses from a continuous wave laser, creating precisely timed pulse sequences. They also play a role in removing unwanted pre-pulses and amplified spontaneous emission, improving the overall quality of the output pulses. This is particularly important in applications such as laser-induced breakdown spectroscopy and medical imaging.